1. Technical Field
The present invention generally relates to communications systems, and particularly relates to scheduling sub-carriers in Orthogonal Frequency Division Multiplexing (OFDM) communications systems.
2. Background
The 3rd Generation Partnership Project (3GPP) is currently developing specifications for new wireless communications systems as part of its “Long Term Evolution” (LTE) initiative. The goals of LTE include very high peak data rates (up to 100 Mbps on the downlink; up to 50 Mbps on the uplink) for mobile users. In order to achieve these goals, LTE as currently planned employs advanced multiple access schemes, adaptive modulation and coding schemes, and advanced multi-antenna technologies.
OFDM technology is a key component of the LTE initiative. Coupled with other evolving technologies, including Multiple-Input Multiple-Output (MIMO), an advanced antenna technology, the LTE initiative promises much higher data rates for mobile wireless users than are currently available, along with more efficient use of radio frequency spectrum. As is well known to those skilled in the art, OFDM is a digital multi-carrier modulation scheme employing a large number of closely-spaced orthogonal sub-carriers. Each sub-carrier is separately modulated using conventional modulation techniques and channel coding schemes. In particular, 3GPP has specified OFDMA (Orthogonal Frequency Division Multiple Access) for the downlink (transmissions from a base station to mobile terminal) and single carrier frequency division multiple access (SC-FDMA) for the uplink (transmissions from a mobile terminal to base station). Both multiple access schemes permit the available sub-carriers to be allocated among several users.
SC-FDMA technology employs specially formed OFDM signals, and is therefore often called “pre-coded OFDM” technology. Although similar in many respects to conventional OFDMA technology, SC-FDMA signals offer a reduced peak-to-average power ratio (PAPR) compared to OFDMA signals, thus allowing transmitter power amplifiers to be operated more efficiently. This in turn facilitates more efficient usage of a mobile terminal's limited battery resources. (SC-FDMA is described more fully in Myung, et al., “Single Carrier FDMA for Uplink Wireless Transmission,” IEEE Vehicular Technology Magazine, vol. 1, no. 3, September 2006, pp. 30-38.)
LTE link resources are organized into “resource blocks,” defined as time-frequency blocks with a duration of 0.5 milliseconds (one slot, or half a sub-frame) and encompassing a bandwidth of 180 kHz (corresponding to 12 sub-carriers with a spacing of 15 kHz). The exact definition of a resource block may vary among LTE and similar systems, and the inventive methods and apparatus described herein are not limited to the numbers used herein. In general, resource blocks may be dynamically assigned to mobile terminals, and may be assigned independently for the uplink (reverse link) and the downlink (forward link). Depending on a mobile terminal's data throughput needs, the system resources allocated to it may be increased by allocating resource blocks across several sub-frames, or across several frequency blocks, or both. Thus, the instantaneous bandwidth allocated to a mobile terminal in a scheduling process may be dynamically adapted to respond to changing conditions.
LTE also employs multiple modulation formats (including at least QPSK, 16-QAM, and 64-QAM), as well as advanced coding techniques, so that data throughput may be optimized for any of a variety of signal conditions. Depending on the signal conditions and the desired data rate, a suitable combination of modulation format, coding scheme, and bandwidth is chosen, generally to maximize the system throughput. Power control is also employed to ensure acceptable bit error rates while minimizing interference between cells.
Efficient utilization of the air interfaces is a key goal of the LTE initiative. A key advantage of the proposed OFDM technologies is the flexibility with which resources may be allocated, or “scheduled”, among multiple users. Theoretically, sub-carriers may be allocated by a base station (or “Node B”) to mobile terminals on an individual basis or in groups; in practice, allocations are typically made on a resource block basis. A variety of scheduling algorithms have been proposed for solving the problem of simultaneously serving multiple users in LTE systems. In general terms, scheduling algorithms are used as an alternative to first-come-first-served queuing and transmission of data packets. As is well known to those skilled in the art, simple scheduling algorithms include round-robin, fair queuing, and proportionally fair scheduling. If differentiated or guaranteed quality of service is offered, as opposed to best-effort communication, weighted fair queuing may be utilized.
Channel-dependent scheduling may be used to take advantage of favorable channel conditions to increase throughput and system spectral efficiency. For example, in an OFDM system, channel quality indicator (CQI) reports, which typically indicate the signal-to-noise ratio (SNR) or signal-to-noise-plus-interference ratio (SINR) measured or estimated for a given channel, may be used in channel-dependent resource allocation schemes. The simplest scheme, conceptually, is to select a mobile terminal having a highest priority, whether based on fairness, quality-of-service guarantees, or other decision metric, and to allocate some number of sub-channels with the highest measured or estimated SINRs to the selected mobile terminal. This approach exploits the frequency diversity inherent to a multi-user OFDM system. Since different mobile terminals observe different frequency-dependent fading profiles, channel-dependent scheduling tends to allocate portions of the overall available bandwidth in a more efficient manner than arbitrary allocation of bandwidth chunks.
As was discussed above, sub-carriers or resource blocks may be allocated on an individual basis. However, CQI- or SINR-based allocation of individual sub-carriers or resource blocks will often lead to distributed allocations of resources, e.g., allocation of two or more widely separated resource blocks to a single mobile terminal. This may be undesirable for a number of reasons. First, generating the transmitted signal may be complicated, particularly in the case of mobile SC-FDMA transmitters, by distributed allocation of sub-carriers. Second, dynamically scheduled allocations must typically be reported by the base station to the mobile terminal. Reporting allocations of several arbitrarily spaced resource blocks to each of several mobile terminals can consume valuable link resources that are better used for other purposes.